Generator systems that are installed in aircraft may include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes permanent magnets on its rotor. When the PMG rotates, AC currents are induced in stator windings of the PMG. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current. This DC current next is provided to stator windings of the exciter. As the rotor of the exciter rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the rotor of the exciter rectify this three-phase AC current, and the resulting DC currents are provided to the rotor windings of the main generator. Finally, as the rotor of the main generator rotates, three phases of AC current are typically induced in its stator windings, and this three-phase AC output can then be provided to a load such as, for example, electrical aircraft systems.
Because the generators installed in aircraft will often be variable frequency generators that rotate in the speed range of 12,000 rpm to 24,000 rpm, large centrifugal forces are imposed upon the rotors of the generators. Given these stressful operating conditions, the rotors of the generators should be carefully designed and manufactured, both so that the rotors are reliable and also so that the rotors are precisely balanced. Improper balancing in particular can result not only in inefficiencies in the operation of the generators, but also in a risk of failure of the generators.
Among the important components in rotors that should be carefully designed and manufactured in order to guarantee robustness and proper balancing of the rotors are the wire coils of the rotors. The centrifugal forces experienced by the rotors are sufficiently strong as to cause bending of the wires of these coils, which over time can result in mechanical breakdown of the wires. Additionally, because the coils are assemblies of individual wires that can move to some extent with respect to one another and with respect to the remaining portions of the rotors, the coils constitute one of the significant potential sources of imbalance within the rotors. The stresses and movement experienced by the wire coils are particularly problematic at the end turns of the coils, at which wires loop from first directions to second directions on the poles supporting the wires.
In order to guarantee robust wire coils and to minimize the amount of imbalance in the rotors that occurs due to the wire coils, the process of initially wrapping the wires of the coils onto the rotors is typically a time-intensive, expensive task. To keep the wrapped coils to within specified tolerances, complicated tooling equipment and an involved process of utilizing shims to guarantee sufficient internal pressure among the wires of the coil are required. Often, the wrapping process involves a significant amount of trial and error before all of the multiple coils on a multi-pole generator are properly configured to have the desired balancing and other characteristics. The difficulty of the wrapping process is greatest at the end turns of the coil wires.
Hence, there is a need for a new system and method for supporting and retaining the wire coils in rotors, particularly at the end turns of the coils. There further is a need for a system and method for end turn retention in which the coil wires at the end turns are positioned accurately and held reliably in position. There additionally is a need for such a system and method whereby the wrapping process is made simpler, more accurate and repetitive, and more cost-effective.